rcserver: improve content-type check

Some libraries use `application/json; charset=utf-8` as their `Content-Type`, which is valid.
However we were not decoding the JSON body in that case, resulting in issues communicating with the rcserver.

fs/rc/rcserver/rcserver.go

@@ -209,8 +209,21 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 	ctx := r.Context()
 	contentType := r.Header.Get("Content-Type")
 
+	var (
+		contentTypeMediaType string
+		contentTypeParams    map[string]string
+	)
+	if contentType != "" {
+		var err error
+		contentTypeMediaType, contentTypeParams, err = mime.ParseMediaType(contentType)
+		if err != nil {
+			writeError(path, nil, w, fmt.Errorf("failed to parse Content-Type: %w", err), http.StatusBadRequest)
+			return
+		}
+	}
+
 	values := r.URL.Query()
-	if contentType == "application/x-www-form-urlencoded" {
+	if contentTypeMediaType == "application/x-www-form-urlencoded" {
 		// Parse the POST and URL parameters into r.Form, for others r.Form will be empty value
 		err := r.ParseForm()
 		if err != nil {
@@ -229,7 +242,13 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 	}
 
 	// Parse a JSON blob from the input
-	if contentType == "application/json" {
+	if contentTypeMediaType == "application/json" {
+		// Check the charset is utf-8 or unset
+		if charset, ok := contentTypeParams["charset"]; ok && !strings.EqualFold(charset, "utf-8") {
+			writeError(path, in, w, fmt.Errorf("unsupported charset %q for JSON input", charset), http.StatusBadRequest)
+			return
+		}
+
 		err := json.NewDecoder(r.Body).Decode(&in)
 		if err != nil {
 			writeError(path, in, w, fmt.Errorf("failed to read input JSON: %w", err), http.StatusBadRequest)

fs/rc/rcserver/rcserver_test.go

@@ -429,6 +429,18 @@ func TestRC(t *testing.T) {
 	"param1": "string",
 	"param2": true
 }
+`,
+	}, {
+		Name:        "json-mixed-case-content-type",
+		URL:         "rc/noop",
+		Method:      "POST",
+		Body:        `{ "param1":"string", "param2":true }`,
+		ContentType: "ApplicAtion/JsOn",
+		Status:      http.StatusOK,
+		Expected: `{
+	"param1": "string",
+	"param2": true
+}
 `,
 	}, {
 		Name:        "json-and-url-params",
@@ -459,6 +471,44 @@ func TestRC(t *testing.T) {
 	"path": "rc/noop",
 	"status": 400
 }
+`,
+	}, {
+		Name:        "json-charset",
+		URL:         "rc/noop",
+		Method:      "POST",
+		Body:        `{ "param1":"string", "param2":true }`,
+		ContentType: "application/json; charset=utf-8",
+		Status:      http.StatusOK,
+		Expected: `{
+	"param1": "string",
+	"param2": true
+}
+`,
+	}, {
+		Name:        "json-mixed-case-charset",
+		URL:         "rc/noop",
+		Method:      "POST",
+		Body:        `{ "param1":"string", "param2":true }`,
+		ContentType: "aPPlication/jSoN; charset=UtF-8",
+		Status:      http.StatusOK,
+		Expected: `{
+	"param1": "string",
+	"param2": true
+}
+`,
+	}, {
+		Name:        "json-bad-charset",
+		URL:         "rc/noop",
+		Method:      "POST",
+		Body:        `{ "param1":"string", "param2":true }`,
+		ContentType: "application/json; charset=latin1",
+		Status:      http.StatusBadRequest,
+		Expected: `{
+	"error": "unsupported charset \"latin1\" for JSON input",
+	"input": {},
+	"path": "rc/noop",
+	"status": 400
+}
 `,
 	}, {
 		Name:        "form",
@@ -498,6 +548,19 @@ func TestRC(t *testing.T) {
 	"path": "rc/noop",
 	"status": 400
 }
+`,
+	}, {
+		Name:        "malformed-content-type",
+		URL:         "rc/noop",
+		Method:      "POST",
+		ContentType: "malformed/",
+		Status:      http.StatusBadRequest,
+		Expected: `{
+	"error": "failed to parse Content-Type: mime: expected token after slash",
+	"input": null,
+	"path": "rc/noop",
+	"status": 400
+}
 `,
 	}}
 	opt := newTestOpt()